After orbiting Jupiter 34 times and surviving four times the amount
of radiation it was design to withstand, the resilient Galileo
spacecraft is finally at the very end of its 14-year mission. To
avoid even the most remote possibility of colliding with a pristine
moon in the jovian system, the out-of-fuel spacecraft will dive
into Jupiter on Sunday, Sept. 21, 2003.

Since its launch in 1989, the sturdy spacecraft traveled more than
4.6 billion kilometers (almost 2.8 billion miles), about the
equivalent of seven times the distance between Earth and Jupiter.
Despite communication problems and a temperamental tape recorder,
Galileo returned 30 gigabytes of data, including 14,000 pictures.

This wealth of information drastically expanded our understanding
of the solar system’s biggest planet and its moons. The mission
was possible because it drew its power from two long-lasting
radioisotope thermoelectric generators provided by the Department of

Asteroids Unveiled

The exciting list of discoveries started even before Galileo was
able to get a close glimpse of Jupiter. As it crossed the asteroid
belt in October 1991, Galileo snapped images of Gaspra, returning
the first ever close-up image of an asteroid. Less then a year later,
the spacecraft got up close and personal with yet another asteroid,
Ida. Images from Ida revealed the asteroid has its own little
"moon," Dactyl, the first known moon of an asteroid.

Location, Location

In 1994 the spacecraft was in the right place at the right time and
made the only direct observation of a comet impacting a planet. It
took images of fragments of comet Shoemaker-Levy 9 crashing into
Jupiter. Images of the impact, which was not visible from
Earth, helped scientists better understand this type of event.

At Jupiter

Galileo began its tour of the jovian system in December 1995.
Carefully designed orbits allowed the spacecraft to observe Jupiter’s
atmosphere, revealing numerous large thunderstorms many times
larger than those on Earth, with lightning strikes up to 1,000 times
more powerful than terrestrial lightning. Data collected by the
descent probe made the first in-place studies of the planet’s clouds
and winds, and it furthered scientists’ understanding of how Jupiter
evolved. The probe also made measurements designed to assess the
degree of evolution of Jupiter compared to the Sun.

As the first spacecraft in long-term residence in jovian orbit,
Galileo also successfully studied the global structure and
dynamics of Jupiter’s magnetic field. Galileo also determined
that Jupiter’s ring system is formed by dust kicked up as
interplanetary meteoroids smash into the planet’s four small inner
moons. Data also showed that Jupiter’s outermost ring is actually
made up of two rings, one embedded within another.

Moons’ Wonders

Galileo extensively investigated the geologic diversity of Jupiter’s
four largest moons: Ganymede, Callisto, Io and Europa. Stunning images
revealed the contrasting and changing surfaces of these moons.

Io has extensive volcanic activity, which is continually modifying
the surface. The heat and the frequency of eruption can be 100
times more than that of Earth, something reminiscent of Earth’s
early days. The similarities make Io an ideal laboratory for the
study of what Earth was like more than 3 billion years ago.

The moon Europa, Galileo unveiled, could be hiding a salty ocean
up to 100 kilometers (62 miles) deep underneath its frozen surface.
Images also reveal ice "rafts" the size of cities that have broken
and drifted apart to create a scalloped and broken surface. There
are also indications of volcanic ice flows, with liquid water
flowing across the surface. These discoveries are particularly
intriguing since liquid water is a key ingredient in the process
that may lead to the formation of life.

The biggest discovery surrounding Ganymede was the presence of a
magnetic field, the first moon of any planet known to have one.
Images of this moon featured a faulted and fractured surface that
demonstrated high tectonic activity. Like Europa and Io, Ganymede
has a metallic core. Galileo magnetic data also provided evidence
that Ganymede might have a liquid-saltwater layer as well.

Galileo determined that, while Callisto doesn’t have a metallic
core, its surface shows evidence of extensive erosion. Data
collected raise the question of whether Callisto’s surface may
also hide an ocean.

Last Dance

Galileo’s own discovery of a likely ocean hidden under Europa’s
surface raises the possibility of life there and concern about
protecting it. For that reason, in its final victory lap the Galileo
spacecraft will dive into the atmosphere of the gaseous planet and
disintegrate. Predictably, some of the spacecraft findings raised
intriguing questions that will have to be answered by future
mission. But Galileo Galilei, the first modern astronomer,
would be immensely proud of the discoveries made by the
spacecraft that carries his name.


  • Development/planning: 12 years
  • Interplanetary cruise: 6 years
  • Orbiting Jupiter: almost 8 years
  • Original idea: October 1977
  • Original name: Jupiter Orbiter Probe mission
  • Launched: Oct. 18, 1989 — onboard the Space Shuttle Atlantis
  • Venus flyby: Feb. 10, 1990
  • First Earth flyby: Dec. 8, 1990
  • Asteroid Gaspra flyby: Oct. 29, 1991
  • Second Earth flyby: Dec. 8, 1992
  • Asteroid Ida flyby: Aug. 28, 1993
  • Witnessed comet Shoemaker-Levy crash into Jupiter: July 1994
  • Arrival at Jupiter: Dec. 7, 1995
  • Number of orbits around Jupiter: 35
  • End of mission: Sept. 21, 2003
  • Total distance traveled: (from launch to impact) 4,631,778,000 kilometers (2,878,053,500 miles)
  • Propellant used: (not counting the fuel for the shuttle) 925 kilograms (246 gallons)
  • Data returned: more than 30 Gigabytes
  • Pictures returned: about 14,000
  • Staff: up to 300